SDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical Network) standards evolved originally for use in a voice network. SDH is a European version of a standard that is substantially the same as the SONET standard developed in North America. SDH/SONET contains connection oriented synchronous TDM circuit switching technology. The SDH/SONET configured network runs at the same clock domain (e.g., every section of the network can be traced to a primary clock reference). The network allocates fixed bandwidth time slots for each circuit. The SDH/SONET architectures are connection based protocols in that there is a physical circuit arrangement between ports in a switch to establish an end to end path. The digital transitions in signals traveling through an SDH/SONET network occur at the same rate, however there may be a phase difference between the transitions of any two signals caused by time delays or jitter in the transmission system.
Ethernet evolved primarily as a data network. In contrast to SDH/SONET, Ethernet is a connectionless asynchronous Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) packet switching technology. The Ethernet architecture does not rely on a single clock domain like the SDH/SONET architecture. The Ethernet architecture sends a series of packets across the network containing data. Whenever a packet needs to be sent, the transmitter will try to transmit the packet. The Ethernet architecture is also connectionless in that the packets travel from node to node within the network without establishing a logical or physical circuit. The end to end path is discovered through a process called “Bridging”. Ethernet is fundamentally a Local Area Networking (LAN) technology.
SDH/SONET networks provide reliable, guaranteed available bandwidth, low jitter connections. These characteristics are required for voice quality networks. SDH/SONET, however, is bandwidth inefficient and has a higher overhead than many other network architectures. Ethernet networks, in contrast, provide lower reliability best effort delivery, and low cost bandwidth connections. These characteristics are suitable for data quality networks. Ethernet has non-guaranteed transmission and low overhead and supports fewer operational functions than SDH/SONET. In SDH/SONET, once the circuit is established, bandwidth is allocated for an application and cannot be used by any other application, even if the original application is not using the bandwidth. In Ethernet, applications only use bandwidth when they need the bandwidth to transmit packets.
In SDH/SONET networks, Operations, Administration, Maintenance, and Provisioning (OAM&P or OAM) functionality is known. OAM&P involves all aspects of the day-to-day operations and fault detection in all parts of the SDH/SONET network. SDH/SONET standards define OAM&P procedures to detect the problems and resolve them. These can include configuration issues, performance statistics, network management, customer support, trouble tracking, performance evaluation, configuration management, technical support, and billing. SDH/SONET Standards define overhead bits in the SDH/SONET frame structure to carry OAM&P information, alarms, and commands; and extensive performance statistics and monitoring information.
In SDH networks OAM functions are performed by three sets of SDH overhead octets/bytes. These are path, multiplex section, and regenerator section overhead octets. SDH Section Overhead (SOH) bytes are classified into Regeneration Section Overhead (RSOH) which terminates at regenerator functions and Multiplex Section Overhead (MSOH) which passes transparently through the regenerators and is terminated where the Administrative Unit Group (AUG-N) are assembled and disassembled. SDH Path Overhead (POH) bytes provides for integrity of communication between the point of assembly of Virtual Container (VC) and its point of disassembly. Two categories of Virtual Container POH have been identified. Higher order Virtual container POH and Lower order Virtual Container.
In SONET networks OAM&P functions are performed by three sets of SONET overhead octets/bytes. There are path, line, and section overhead bytes. SONET Transport Overhead (TOH) bytes are classified into Section Overheard (SOH) which terminates at Section Layer functions and Line Overhead (LOH) which passes transparently through the Section Layer and is terminated where the SONET (SPE) payloads are assembled and disassembled. SONET Path overhead (POH) provide end-to-end OAM&P functions. The POH is passed unchanged through the SONET Line, Section, and Physical Layers. SONET POH bytes are defined for SONET Transport Signal (STS) path and Virtual Tributary (VT) low rate path levels. SONET Physical layer is a transport layer and does not have overhead associated with it.
SDH and SONET Standards are similar in functionality but have different terminology. The mapping of terminology of the two standards is well known in the industry.
In voice networks, SDH/SONET OAMP Standard functionality provides the architecture for administration, configuration issues, performance statistics, network management, customer support, trouble tracking, performance evaluation, configuration management, technical support, and billing. In order for data networks to be able to support carrier operations requirements, the data network must be able to provide the same level of OAMP support. Ethernet is the most common data network data link layer protocol. There is no Ethernet standard to provide OAMP functionality.
In Ethernet networks, several working groups are working on trying to define Ethernet standards to support various levels and forms of OAMP support in Ethernet.
The IEEE 802.3ah Ethernet First Mile (EMF) OAM group, which deals only with the first mile Ethernet support, has two proposals that are being debated. One proposal is to use “slow protocol” MAC frames to support limited OAM functionality for a single Ethernet link. Another proposal is to add additional OAM field in front of every Ethernet frame at the Physical Layer.
The Metro Ethernet Forum (MEF) group also has proposals to support OAM using MPLS protocol. The preliminary proposal is recommending using user/data Ethernet packets with specific VLAN tag to define MEF Ethernet OAM information.
The ITU-T Q.3 SG13 is also studying various options to support OAM in Ethernet networks. It is also working with EFM. It has only defined the requirements so far.
Most of the above mentioned technologies solve limited functionality for Ethernet. Typically, they support only limited OAMP functionality and, also are limited to Ethernet Links, such as link failure alarms and a small set of performance monitoring statistics. They address only a subset of the capabilities as defined by the SDH/SONET OAMP standard. Using the same techniques as used in existing SDH/SONET networks, allows for easier migration and coexistence of both SDH/SONET and Ethernet networks. One network management can support both networks transparently, and not need to retrain network operators in new operations methods.